The present invention relates to radiotelephone communication systems and more particularly to a method and apparatus for adjusting a measured signal strength value to compensate for receiver-to-receiver variations in signal strength measurement which occur due to differences in system components.
In a cellular telephone communications system operating in accordance with the GSM standard, a base station radio receiver (RRX) measures the strength of a radio signal received from a mobile station to produce a radio signal strength information (RSSI) signal in the form of an eight-bit digital code. The RSSI signal for a given mobile station will vary as that mobile moves within a cell and as it crosses into a new cell. It is known in the art to use RSSI signals to control the handover process, i.e., to determine when a mobile station's established telephone call should be switched from one cell to another.
In accordance with the GSM standard set by ETSI and CEPT, the RSSI signal is allowed to deviate by only .+-.4 dB from the actual strength of the incoming antenna signal. This tolerance is relatively tight compared with the tolerances of previous standards. As a matter of fact, other standards provide for the RSSI signal to take the form of an analog voltage level without any tolerance limitations.
In a base station site, the radio signal received by the antenna must propagate through a bandpass filter (RXBP), an antenna signal amplifier (RXDA), and a divider (RXD) before entering the radio receiver RRX. As the received radio signal follows its path through the base station site, ripples larger than the standard's RSSI signal tolerance may be generated that cause the RSSI signal to deviate from the above-described standard. To compensate for these overriding ripples and thus to restore the degraded RSSI signal to a level that meets the standard's requirements, analog circuits in the receiver usually have to be adjusted. The amount of RSSI signal adjustment that is required varies from one radio receiver to another, however, because each radio receiver's components affect the incoming signal differently from the components of other radio receivers.